Hurley etal
hydrophones



United States Patent 3,258,740 HYDROPHONES Thomas P. Hurley, Bennington,Vt., and Noel C. Sears, Becket, Mass., assiguors to Sprague ElectricCompany, North Adams, Mass., a corporation of Massachusetts Filed Oct.16, 1964, Ser. No. 404,385 2 Claims. (Cl. 340-) This is acontinuation-in-part of application Serial No. 307,934, filed September10, 1963.

This invention relates to transducers for converting electricaloscillations into traveling compressional waves in fluids andvice-versa, and in particular is concerned with transducers employingelectro mechanically responsive ceramics such as barium titanate.

The principal object of the invention is to provid an underwatertransducer of increased sensitivity.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same become better understood byreference to the following description when considered in conjunctionwith the accompanying drawing wherein:

FIGURE 1 is a perspective exploded view of the piezoelectric elementshowing the position of the contacts of the cable to the inner and outerelectrodes and also showing the end caps of the present invention; and

FIGURE 2 is a perspective view of th encapsulated element.

The hydrophone of the present invention comprises a piezoelectric tubehaving metallic electrodes on its inner and outer surfaces. One lead ofa cable is in electrical communication with the inner electrode, and ata point adjacent this connection the other lead is in electricalcommunication with the outer electrode. The cable circumscribes theperiphery of the tube at least once, said cable being tangent to saidtube at a point remote from the lead connections. Rigid, soundtransparent end caps close the ends of said tube, said rig-id end capsbeing electrically insulated from said tube by thin gaskets. The endcaps, circumscribing cable, including small lengths of said cable whereit passes itself at the point of tangency, and the piezoelectric tubeare encapsulated in a sound transparent elastomer.

Referring to the drawing, FIGURE 1 shows a hollowcylindricalpiezoelectric element 10 having an inner 11 and outer 12 metallicelectrode. The inner electrode has a bridge or passover 13. Anonelectroded area 14 separates the bridge from th outer electrode. Onelead of a twin lead cable 15 is shown soldered at 16 to the bridge orpassover 13, While the other lead is soldered at 17 to the outerelectrode 12. Rigid, sound transparent end caps 18 and 19, insulated bythin, sound transparent gaskets 20 and 21 are adapted to close the endsof piezoelectric tube 10.

FIGURE 2 shows the unit of FIGURE 1 encapsulated in a sound transparentelastomer 22. This encapsulation extends along small lengths of the leadcable as shown at 23.

The use of the rigid unconnected sound transparent end caps of thepresent invention have been found to increase the sensitivity of thesubject devices by about 3 db. It is essential that the end caps bemechnaically independent of one another, i.e. they must not bephysically interconnected other than through the medium of the flexibleencapsulating material. It is believed that enhancement ice ofsensitivity resulting from said independently operating end caps occursbecause the transducer operates as a volume element. In other words thesound pressure wave actuates the active cylindrical element both on itsperiphery and on its ends simultaneously. These two modes ofpiezoelectric activity act to reinforce each other.

The end caps can be made of practically any material which is rigid andsound transparent. Examples are rigid plastics, wood, ceramic, metal,etc., with the latter being preferred. Particularly effective end capshave been made of brass. The external surface of the end cap ispreferably flat for maximum effectiveness.

The piezoelectric material can be any of the prior art materials,preferably lead titanate-zirconate or the same modified with knownadditives. The electrode metal can be any of the commonly usedmaterials, e.g., silver, the platinum metals, etc. The leads may beattached to the electrodes by any means which will result in electricalcommunication, e.g., solder, conductive adhesive, etc. The soundtransparent elastomer includes natural, reclaimed, vulcanized, syntheticrubber or polyurethanes of a Shore A durometer reading of 30 to 80.

The elastomer must also be sound transparent and preferably matched tothe liquid environment in which the unit is to be used. The elastomerjacket material will act as a waterproof coating and more particularlyit provides a pressure-actuated seal about th cable at the points ofegress of the cable from the unit. While not essential for successfulsealing it has been found that the tapered design, as shown at 23 inFIGURE 2, has greater lead flexibility which permits a smaller diameterstowage stack.

The thin, sound transparent gasket which separates the end caps from thpiezoelectric tube must be a material which does not itself effect abond between the cap and tube. It also must not bond to theencapsulating material or be wetted thereby or dissolved therein. If thegasket material possessed any of these prohibited characteristics, theend caps would actually or in effect be bonded to the transducer and theincrease in sensitivity would be lost. Teflon or polytetrafiuoroethyleneis an example of a gasket material possessing the requiredcharacteristics. For the purposes of this invention the term nonbondingas descriptive of the gasket means that it will not bond to thetrandsucer, the end caps or the encapsulating elastomer.

In use, a plurality, i.e., two or more, of assemblies is usuallyemployed. They are linked together by means of the cable. The bottomassembly in the string tends to be misaligned with respect to the otherassembly or assemblies if a compensating weight or dummy assembly is notsuspended from this bottom assembly. Therefore, it is preferred that thebottom assembly be provided with a trailing end of cable so that thecompensating weight can be attached thereto.

Although the external design of the assembly has been shown to haveroughly the shape of a cylinder or hockey puck, it is clear that otherdesigns, which would be of greater advantage in the constantly movingsea, are contemplated.

As is evident from the foregoing, the invention is not to be limited toformation of the rather specific illustrative device. Modifications andvariations, particularly as to shape, as well as the substitution ofequivalents, may be made without departing, from the spirit of theinvention as defined in the appended claims.

What is claimed is:

1. A device of the type described comprising a piezoelectric tube havingmetallic electrodes on its inner and outer surfaces; one lead of a cableis in electrical communication with the inner electrode, and at a pointadjacent this connection another lead is in electrical communicationwith the outer electrode; the cable circumscribes :the periphery of thetube at least once, said cable Fbeing tangent to said tube at a pointremote from the lead connect-ions; the ends of said tube are closed byrigid, sound transparent end caps, said end caps being physicallyseparated from the ends of the tub by thin, nonbonding, soundtransparent gaskets; said tube, said circumscribing cable, includingsmall lengths of said cable where it passes itself at the point oftangency, and said rigid end caps being encapsulated in a soundtransparent elastomer.

2. The device of claim 1 wherein the end caps are metal and the gasketsare polytetrafiuoroethylene.

References Cited by the Examiner CHESTER L. JUSTUS, Primary Examiner.

G. M. FISHER, Assistant Examiner.

1. A DEVICE OF THE TYPE DESCRIBED COMPRISING A PIEZOELECTRIC TUBE HAVINGMETALLIC ELECTRODES ON ITS INNER AND OUTER SURFACES; ONE LEAD OF A CABLEIS IN ELECTRICAL COMMUNICATION WITH THE INNER ELECTRODE, AND AT A POINTADJACENT THIS CONNECTION ANOTHER LEAD IS IN ELECTRICAL COMMUNICATIONWITH THE OUTER ELECTRODE, THE CABLE CIRCUMSCRIBES THE PERIPHERY OF THETUBE AT LEAST ONE, SAID CABLE BEING TANGENT TO SAID TUBE AT A POINTREMOTE FROM THE LEAD CONNECTIONS; THE ENDS OF SAID TUBE ARE CLOSED BYRIGID, SOUND TRANSPARENT END CAPS, SAID ENDS CAPS BEING PHYSICALLYSEPARATED FROM THE ENDS OF THE TUBE BY THIN, NONBONDING, SOUNDTRANSPARENT GASKETS; SAID TUBE, SAID CIRCUMSCRIBING CABLE, INCLUDINGSMALL LENGTHS OF SAID CABLE WHERE IT PASSES ITSELF AT THE POINT TOTANGENCY, AND SAID RIGID END CAPS BEING ENCAPSULATED IN A SOUNDTRANSPARENT ELASTOMER.